Tacitus the Pretty-Good Historian

Historians in all ages become liable through their profession to certain maladies or constraints. They cannot help making persons and events more logical than reality; they are often paralysed by tradition or convention; and they sometimes fall a prey to morbid passions for a character or an idea. Tacitus in his account of Tiberius betrays each and all of these three infirmities. From a fourth, the most insidious and pernicious, he is wholly exempt.

Historians know the verdict in advance, they run forward with alacrity to salute the victors and chant hymns to success. A chorus of hierophants or apologists acclaimed the Roman Caesars - but not uncontradicted, and not earning good fame. The Roman senator despised them. On his guard against specious or shabby concessions, he refused to condone violence because, having succeeded, it became respectable; and, if nothing short of authoritarian government ensured peace and stability, he accepted it without rejoicing or any subservience.

Syme, Tacitus, Vol.1,Ch.33

In other words - more or less - Tacitus sometimes (1) oversimplified event chains & causality, (2) got bogged down narrating political stuff, and (3) revealed clearly his own feelings about tyranny... and yet he did NOT propagandize, he did NOT seek to excuse embarrassing things, and he did NOT force his own written account into celebrating pet views (either his own or his subjects')!

That is, for Tacitus, early Imperial Rome simply was what it was. "Rome at the outset [had been] a city state under the government of kings" which soon enough adopted "liberty and the consulate" as permanent institutions, and yet "Dictatorships were always a temporary expedient". Temporary, that is, until Octavian's fellow Triumvirs flamed out, at which point "the sole remedy for his distracted country was government by one man." (Annals 1.1, 9.5)

It seems that being transparent about his own bias actually made Tacitus more free to be fair about things he disliked.